My invention relates to a temperature compensator for a crystal oscillator having a voltage sensitive capacitor or varactor, and particularly to a temperature compensator for controlling the voltage applied to such a voltage sensitive capacitor, and hence for providing improved oscillator frequency stability over a wide range of temperatures.
In radio communication and electronic equipment piezoelectric crystals are used almost exclusively as the frequency determining element. As the technology in this field has expanded, the desired frequency stability of the piezoelectric crystals with respect to temperature has steadily increased. Thus, in oscillators using piezoelectric crystals, it is now fairly common to have a frequency stability of plus or minus five parts per million (.+-.5 ppm) over a temperature range between -30.degree. C and +80.degree. C. (As used herein, frequency stability stated in parts per million indicates the maximum frequency variation from a crystal's center frequency over a given temperature range. Thus, if a crystal is specified to have a center frequency of 8 megahertz and a stability of .+-.2 ppm for a given temperature range, this means that the crystal frequency will not vary more than 2/1,000,000 times 8,000,000 hertz, or .+-.16 Hertz over that temperature range.)
Accordingly, a primary and general object of my invention is to provide a new and improved temperature compensator for a crystal oscillator.
Another and fairly specific object of my invention is to provide a new and improved temperature compensator that can maintain the frequency stability of a crystal oscillator to within .+-.2 ppm over a temperature range of -30.degree. C to +80.degree. C.
Temperature compensators using temperature sensitive resistors (or thermistors) have been provided in the past. Generally, these thermistors cannot be adjusted or trimmed, so that additional and fairly complex circuits must be provided to adjust the compensator for any differences between the thermistors.
Accordingly, another object of my invention is to provide a new and improved temperature compensator that does not require thermistors.
Another object of my invention is to provide a new and improved temperature compensator that can be built in integrated circuit form and provided with external components that can be easily adjusted so that the compensator has the desired compensation characteristics.